1. Field of the Invention
The present invention relates generally to the field of medicine. More specifically, the invention relates to methods of obtaining novel plant compounds having therapeutic uses in mammals.
2. Description of Related Art
Plants are valuable sources for the identification of novel biologically active molecules. One diverse class of molecules which has been identified in plants is the class of saponins. Saponins are high molecular weight compounds comprising glycosides with a sugar moiety linked to a triterpene or steroid aglycone. Triterpene saponins particularly have been the subject of much interest because of their biological properties.
Pharmacological and biological properties of triterpene saponins from different plant species have been studied, including fungicidal, anti-viral, anti-mutagenic, spermicidal or contraceptive, cardiovascular, and anti-inflammatory activities (Hostettmann et al., 1995). Saponins are known to form complexes with cholesterol by binding plasma lipids, thereby altering cholesterol metabolism (Oakenfull et al., 1983). Triterpene glycosides given in feed also have been shown to decrease the amount of cholesterol in the blood and tissues of experimental animals (Cheeke, 1971). Saponins have been found to be constituents of many folk medicine remedies and some of the more recently developed plant drugs.
The triterpene glycyrrhetinic acid, and certain derivatives thereof, are known to have anti-ulcer, anti-inflammatory, anti-allergic, anti-hepatitis and antiviral actions. For instance, certain glycyrrhetinic acid derivatives can prevent or heal gastric ulcers (Doll et al., 1962). Among such compounds known in the art are carbenoxolone (U.S. Pat. No. 3,070,623), glycyrrhetinic acid ester derivatives having substituents at the 3′ position (U.S. Pat. No. 3,070,624), amino acid salts of glycyrrhetinic acid (Japanese Patent Publication JP-A-44-32798), amide derivatives of glycyrrhetinic acid (Belgian Patent No. 753773), and amide derivatives of 11-deoxoglycyrrhetinic acid (British Patent No. 1346871). Glycyrrhetinic acid has been shown to inhibit enzymes involved in leukotriene biosynthesis, including 5-lipoxygenase activity, and this is thought to be responsible for the reported anti-inflammatory activity (Inoue et al., 1986).
Betulinic acid, a pentacyclic triterpene, is reported to be a selective inhibitor of human melanoma tumor growth in nude mouse xenograft models and was shown to cause cytotoxicity by inducing apoptosis (Pisha et al., 1995). A triterpene saponin from a Chinese medicinal plant in the Cucurbitaceae family has demonstrated anti-tumor activity (Kong et al., 1993). Monoglycosides of triterpenes have been shown to exhibit potent and selective cytotoxicity against MOLT-4 human leukemia cells (Kasiwada et al., 1992) and certain triterpene glycosides of the Iridaceae family inhibited the growth of tumors and increased the life span of mice implanted with Ehrlich ascites carcinoma (Nagamoto et al., 1988). A saponin preparation from the plant Dolichos falcatus, which belongs to the Leguminosae family, has been reported to be effective against sarcoma-37 cells in vitro and in vivo (Huang et al., 1982). Soya saponin, also from the Leguminosae family, has been shown to be effective against a number of tumors (Tomas-Barbaren et al., 1988). Oleanolic acid and gypsogenin glycosides exhibiting haemolytic and molluscicidal activity have been isolated from the ground fruit pods of Swartzia madagascariensis (Leguminosae) (Borel and Hostettmann, 1987).
Genistein, a naturally occurring isoflavonoid isolated from soy products, is a tyrosine kinase inhibitor that has been shown to inhibit the proliferation of estrogen-positive and estrogen-negative breast cancer cell lines (Akiyama et al., 1987). Inositol hexaphosphate (phytic acid), which is abundant in the plant kingdom and is a natural dietary ingredient of cereals and legumes, has been shown to cause terminal differentiation of a colon carcinoma cell line. Phytic acid also exhibits anti-tumor activity against experimental colon and mammary carcinogenesis in vivo (Yang et al., 1995). Some triterpene aglycones also have been demonstrated to have cytotoxic or cytostatic properties, i.e., stem bark from the plant Crossopteryx febrifuga (Rubiaceae) was shown to be cytostatic against Co-115 human colon carcinoma cell line in the ng/ml range (Tomas-Barbaren et al., 1988).
While the previous reports have identified triterpene compounds which have any of a number of uses, there still is a great need in the art for the identification of novel biologically active triterpene compounds. Many of these compounds are toxic to normal mammalian cells. Still further, the biological activities of previously identified triterpenes vary widely and many posses limited or varying degrees of efficacy in the treatment of any given human or mammalian condition. The great diversity of different triterpenes which have been identified and the great range of differences and unpredictability in the biological activities observed among even closely related triterpene compounds, underscores the difficulties which have been encountered in obtaining triterpenes which are potential therapeutic agents. Achieving the difficult goal of identifying novel triterpenes with beneficial biological activities could provide entirely new avenues of treatment for a diverse set of human ailments in which therapeutic options currently are limited.